In recent years, electric vehicles and hybrid vehicles are beginning to prevail, which include devices such as motors and batteries, and travel by driving the motors with power stored in the batteries. The electric vehicles charge their batteries with power supplied from external power supply devices. As for the hybrid vehicles, plug-in hybrid vehicles have been practically used, which allow their batteries to be charged by external power supply devices. The external power supply devices are power supply devices installed in ordinary houses or facilities such as commercial power supply stations. When a power supply device supplies power to a vehicle, a plug at an end of a charging cable connected to the power supply device is connected to a power supply port which is provided in the vehicle as a power receiving connector. Then, power is supplied from the power supply device to the vehicle via a power supply line contained in the charging cable, and thereby the battery is charged.
Not only the power supply line but also other lines such as a grounding line, a control line, and the like are contained in the charging cable. The control line is a line used for transmission of a control signal such as a control pilot signal or the like used for control of power supply to the power storage device. By transmitting and receiving the control signal between the power supply device and the vehicle via the control line, various states are detected such as the connection state of the charging cable, whether charging is possible or not, the state of charging, and the like, and charging control is performed according to the detected states.
Further, for practical use of vehicles that need external power supply, such as electric vehicles and hybrid vehicles, a communication function is required which allows a vehicle and a power supply device to transmit and receive information for charging control, and communication information for management of the amount of charge or accounting, or the like.
Thus, standardization of communication such as power line communication has been progressed in which a communication signal is superposed on a power supply line as a medium to allow communication between a vehicle and a power supply device. As for the method of transmitting/receiving a communication signal, in addition to the power line communication, standardization of communication such as inband communication has also been progressed in which a communication signal is superposed on a control signal using a control line as a medium so as to be transmitted and received between a vehicle and a power supply device (refer to Non-Patent Literature 1, for example).
In the communications such as the power line communication and the inband communication, a superposition/separation unit using a transformer having a first coil and a second coil is connected to lines such as a power supply line, a grounding line, a control line, and the like. The superposition/separation unit is connected to a communication device via a signal line, and superposes and separates, on and from the lines, a communication signal input to and output from the communication device, thereby allowing a vehicle and a power supply device to perform communication.
FIG. 5 is a circuit diagram showing a configuration regarding a transmission circuit included in a communication device in a conventional communication system. In FIG. 5, reference numeral 1000 denotes the transmission circuit included in the communication device. The transmission circuit 1000 includes two line drivers 1001 and 1001 each outputting a communication signal. The line drivers 1001 and 1001 are connected to a superposition/separation unit 1003 by two signal lines 1002 and 1002 that transmit the communication signals. In addition, resistance elements 1004 and 1004 and capacitance elements 1005 and 1005 are interposed in the two signal lines 1002 and 1002, respectively. The resistance elements 1004 and 1004 are current limiting resistors having a peak current suppressing function. The capacitance elements 1005 and 1005 are capacitors having a DC component blocking function. A high-level or low-level driver enable signal (Driver Enable) is input to the line drivers 1001 and 1001 from a not-illustrated control circuit. Each of the line drivers 1001 and 1001 is designed to operate only when transmitting a communication signal, and is switched from an idle state where it cannot transmit the communication signal to an operating state where it can transmit the communication signal when the driver enable signal is switched from a low level to a high level. In addition, when the driver enable signal is switched from the high level to the low level, each of the line drivers 1001 and 1001 is switched from the operating state to the idle state.